DC SWITCH

Description

[0001] The invention relates to a DC voltage switch for coupling a DC voltage device or DC voltage branch with a positive and a negative conductor to a DC voltage bus and an associated method.

[0002] Direct current refers to voltages up to 1500 volts. DC voltages up to this level are also referred to as low voltage. More specifically, DC voltage refers to voltages greater than extra-low voltage (120 volts DC). Direct current refers specifically to voltages between 400 and 800 volts.

[0003] DC circuits are circuits for currents, particularly nominal or maximum currents, from 2 to 1000 amperes; more specifically, for currents from 2 amperes to 400 amperes or 200 amperes.

[0004] A DC bus is a system with at least two conductors, one positive conductor and one negative conductor, supplied with DC voltage from at least one energy source. A (DC) device, e.g., a DC consumer, load, inverter, combined energy sink or energy source, pure (additional) energy source, etc., is connected to the DC bus via a DC branch. Multiple DC devices can also be connected to a DC branch.

[0005] A DC device is specifically a device with a power output of 1 kilowatt to 500 kilowatts.

[0006] Meanwhile, DC circuits, also known as DC networks or low-voltage DC networks, are increasingly being developed and constructed, which usually have a DC bus with DC branches.

[0007] The DC voltage feeders, also known as load feeders, are usually protected by a DC switch. These DC switches have a switching device.

[0008] A schematic diagram of a DC circuit, DC network, low-voltage DC network or DC network with a DC bus DCB and several DC branches DCA1, DCA2, DCA3 with DC switches S1, S2, S3 shows Figure 1 .

[0009] Figure 1shows the principle of a DC voltage source EQ connected to a DC voltage bus DCB via a feed switch S4. A first DC voltage branch DCA1 with at least one first DC voltage device G1 is connected to the DC voltage bus DCB via a first DC voltage switch S1; in a similar manner, a second DC voltage branch DCA2 with at least one second DC voltage device G2 is connected via a second DC voltage switch S2; and a third DC voltage branch DCA3 with at least one third DC voltage device G3 is connected via a third DC voltage switch S3.

[0010] Additional DC switches, DC feeders, and DC devices may be provided. These DC switches may be outgoing switches. These DC devices typically have an electrical capacity that can store significant amounts of energy.

[0011] If an error occurs in the DC circuit according to Figure 1 If a fault occurs, e.g., a short circuit in the first DC branch DCA1 at fault location F1, which is located between the first DC switch S1 and the first (DC) device G1, the short circuit there is fed from the energy source EQ as well as from the surrounding DC branches, in this example, the second and third DC branches DCA2, DCA3, or the energy sources or capacitances (of the devices) located there. This leads to a high current in the associated switching device, in this example, the second or third DC switches S2, S3, which can trigger / trip with a shutdown.

[0012] The second and third devices G2, G3, or their respective capacitances, can supply a high (fault) current. For example, if the second device G2 has a low rated current, the second DC switch S2 is correspondingly small and can interrupt the current flow, even if the fault occurred in another branch.

[0013] In principle, the surrounding DC switches (S2, S3) should impede the current flow from the respective DC feeder or load feeder to the short circuit F1 as little as possible so that the DC switch, in the example the first DC switch S1, trips safely.

[0014] It is also important that the surrounding DC switches (S2, S3) do not trigger, so that a so-called selective shutdown of the fault (by S1) occurs.

[0015] As shown above, this is not always guaranteed, especially when the surrounding DC switches contain switching devices with semiconductor switching elements. These semiconductor switching elements usually have a saturation current that is lower than the short-circuit current in the event of a fault, and thus have a current-limiting effect. Furthermore, this saturation current can only be carried by the semiconductor switching element for a short time, usually in the single-digit ps range. Therefore, to protect the semiconductor switching element, the DC switches must switch off without taking the required selectivity into account (self-protection), i.e., interrupt the current flow.

[0016] For example, WO 2014 / 139559 A1 discloses a high-performance switch for interrupting DC circuits with two current paths for high power, wherein at least two transistors connected anti-serially in one of the current paths are provided such that each transistor can block or switch on exactly one of the two possible current directions.

[0017] Until now, this problem has been solved by either using electromechanical switching devices or by over-dimensioning the DC switches, which is expensive or not economical.

[0018] The object of the present invention is to improve the switching behavior of DC switches to achieve selectivity, in particular to provide a solution to the aforementioned problem for DC switches with semiconductor-based switching devices.

[0019] This problem is solved by a DC voltage switch having the features of patent claim 1 or a method having the features of patent claim 16.

[0020] According to the invention, a DC voltage switch is proposed for coupling a DC voltage branch provided for at least one DC voltage device with a positive and a negative conductor to a DC voltage bus, wherein a forward direction of the electric current is defined by a current flow from the positive conductor of the DC voltage bus DCB via the DC voltage switch to the positive conductor of the DC voltage branch and from the negative conductor of the DC voltage branch via the DC voltage switch to the negative conductor of the DC voltage bus.

[0021] A reverse direction of the electric current is therefore defined from the positive conductor of the DC branch via the DC switch to the positive conductor of the DC bus and from the negative conductor of the DC bus via the DC switch to the negative conductor of the DC branch.

[0022] The DC switch has: a semiconductor-based, electronically controllable switching device, a DC bus-side voltage sensor provided upstream of the switching device for determining the DC bus-side voltage level, a DC branch-side voltage sensor provided downstream of the switching device for determining the DC branch-side voltage level, a current sensor for determining the current level and the current direction, a control device connected to the switching device, the voltage sensors and the current sensor.

[0023] The control device is designed in such a way that the direction of the current and the current level are determined, in particular the direction and current level are determined periodically, if a first threshold value (current threshold value) of the current level is exceeded, the current flow through the switching device is interrupted, if the determined direction of the current is the reverse direction when the first threshold value of the current level is exceeded: after the current flow is interrupted, the voltage level on the DC bus side is compared with the voltage level on the DC branch side, if the voltage difference is less than a voltage difference value, the switching device is switched to conductive.

[0024] This has the advantage that if the DC switch is accidentally switched off due to excessive current flow, it will only be switched on again if the current (current level) causing the switch-off did not flow to the DC device, but from the DC device to the DC bus. This means that a fault does not exist in the DC branch of the DC switch, but rather on the DC bus or another DC branch. Furthermore, if the voltage level on the DC bus has returned to a value that is roughly equivalent to the voltage level in the DC branch. In such a case, it can be assumed that a faulty branch has been switched off and the voltage on the DC bus has normalized, so that fault-free DC branches that accidentally switched off can continue to be supplied as intended.

[0025] Advantageous embodiments of the invention are specified in the subclaims.

[0026] In an advantageous embodiment of the invention, the switching device has at least one, in particular two, semiconductor switching elements carrying the current flow on the plus or minus conductor side.

[0027] This has the particular advantage of providing a particularly simple solution for a switching device.

[0028] In an advantageous embodiment of the invention, the semiconductor switching element is a bipolar transistor with an insulated gate electrode, a metal oxide semiconductor field effect transistor or a gallium nitride transistor.

[0029] This has the particular advantage of providing a simple solution for the semiconductor switching elements of the switching module.

[0030] In an advantageous embodiment of the invention, a diode is connected in parallel to the semiconductor switching element, in particular with its forward direction opposite to the forward direction of the semiconductor switching element.

[0031] This has the particular advantage of providing a simple solution for unidirectional semiconductor switching elements, especially when two unidirectional semiconductor switching elements are connected in series.

[0032] In an advantageous embodiment of the invention, the diode has a low forward voltage, in particular it is a mains diode or Schottky diode.

[0033] This has the particular advantage that there is a particularly low voltage drop in the reverse direction, which on the one hand results in low power loss and on the other hand provides a maximum current in the reverse direction to increase selectivity.

[0034] In an advantageous embodiment of the invention, the DC voltage switch is arranged in a housing.

[0035] This has the particular advantage that a compact DC switch is available in one housing.

[0036] In an advantageous embodiment of the invention, the housing has a positive conductor input terminal, a negative conductor input terminal, a positive conductor output terminal, and a negative conductor output terminal. The input terminals are connectable to the DC bus. The output terminals are connectable to the DC branch.

[0037] The switching device connects the positive wire input terminal to the positive wire output terminal or the switching device connects the negative wire input terminal to the negative wire output terminal.

[0038] In one variant, switching devices can be provided in the positive and negative conductors.

[0039] This has the particular advantage of providing a simple solution for a compact DC voltage switch.

[0040] In an advantageous embodiment of the invention, the terminals not having the switching device are connected to one another by an electrical line.

[0041] This has the particular advantage of providing a simple solution, particularly a single-pole switching solution, for a DC voltage switch.

[0042] In an advantageous embodiment of the invention, the voltage difference value is 10 volts.

[0043] This has the particular advantage that excessive compensating currents between the DC branch and the DC bus are minimized or the voltage difference exceeds a high level.

[0044] In an advantageous embodiment of the invention, the current sensor is a sensor based on the Hall effect.

[0045] This has the particular advantage of providing a simple solution for determining the magnitude and direction of the current.

[0046] In an advantageous embodiment of the invention, the control device is further designed such that when the current increases by a value exceeding a current increase threshold, the current flow through the switching device is interrupted.

[0047] This has the particular advantage that an additional protection criterion of the DC voltage switch is provided for targeted triggering or self-protection.

[0048] In an advantageous embodiment of the invention, a power supply, energy storage or a connection for an external energy supply is provided for the control device.

[0049] This has the particular advantage that a power supply is provided after the DC voltage switch is triggered.

[0050] In an advantageous embodiment of the invention, the control device comprises a microprocessor.

[0051] This has the particular advantage of enabling particularly convenient and variable control of the DC voltage switch.

[0052] All embodiments, both in a dependent form referring back to patent claim 1 or 16 and referring back only to individual features or combinations of features of patent claims, result in an improvement of a DC switch for improving selectivity in a DC voltage network. This allows, in particular, devices of different power classes to be operated on a common DC voltage bus.

[0053] The described properties, features and advantages of this invention and the manner in which they are achieved will become clearer and more clearly understandable in connection with the following description of the embodiments, which are explained in more detail in connection with the drawings.

[0054] The accompanying drawing shows: Figure 1 a schematic diagram of a DC circuit with a DC bus and several DC branches with DC switches, Figure 2 a representation of a DC switch with DC bus, DC branch and DC device according to the invention, Figure 3 an example of a switching device with semiconductor switching elements.

[0055] Figure 1shows a schematic diagram of a DC circuit with a DC bus and several DC branches with DC switches according to the state of the art, as already described at the beginning.

[0056] Figure 2 a DC voltage switch Sx, such as the first, second or third switch S1, S2, S3 according to Figure 1 could be used with a housing GEH. The housing GEH has a positive conductor input terminal PE, a negative conductor input terminal ME, a positive conductor output terminal PA, and a negative conductor output terminal MA.

[0057] The input terminals PE, ME are connected to the DC bus DCB, which has a positive conductor DCP (+) and a negative conductor DCN (-).

[0058] The output terminals PA, MA are connected to a DC voltage branch DCA, which in turn is connected to at least one DC voltage device Gx, for example a first, second or third device G1, G2, G3 according to Figure 1 .

[0059] The DC voltage switch Sx has a switching device SCH.

[0060] Internally in the housing, the switching device SCH connects either the positive conductor input terminal PE with the positive conductor output terminal PA, as shown, or the negative conductor input terminal ME with the negative conductor output terminal MA.

[0061] Alternatively, two switching devices, a first and a second switching device, can be provided. The first switching device is provided for the positive conductor terminals and the second switching device is provided for the negative conductor terminals.

[0062] According to Figure 2the terminals that do not have the switching device SCH are connected to each other by an electrical line; in the example, the negative conductor input terminal ME is connected to the negative conductor output terminal MA by the electrical line.

[0063] In front of the switching device SCH, i.e. on the DC bus side DCB, a DC bus side voltage sensor U1 is provided between the positive and negative conductors to determine the DC bus side voltage level.

[0064] After the switching device SCH, ie on the DC branch DCA side, a DC branch side voltage sensor U2 is provided between the positive and negative conductors to determine the DC branch side voltage level.

[0065] A current sensor I is provided in the positive or negative conductor, before or after the switching device SCH, to determine the current magnitude and direction. The current sensor can be a Hall effect sensor.

[0066] The switching device SCH is a semiconductor-based, electronically controllable switching device SCH. It can have at least one semiconductor switching element that carries the current flow (on the positive or negative conductor side, depending on the position). In particular, two semiconductor switching elements can be provided that carry the current flow (on the positive or negative conductor side, depending on the position). The semiconductor switching element can be a bipolar transistor with an insulated gate electrode, a metal oxide semiconductor field-effect transistor, or a gallium nitride transistor.

[0067] A diode can be connected in parallel with the semiconductor switching element, in particular with its forward direction opposite to the forward direction of the semiconductor switching element. The diode can have a low forward voltage and can be, in particular, a line diode or Schottky diode.

[0068] A control device SE is provided, which is connected to the switching device SCH, the voltage sensors U1, U2, and the current sensor I. The control device SE can have a microprocessor.

[0069] The control device can be powered by a power supply (not shown). The power supply can be connected to the DC bus input terminals and / or the DC branch output terminals.

[0070] Alternatively or additionally, an energy storage device, such as a supercapacitor, accumulator, or battery, can be provided to supply power to the control device SE in the event of a trip or a short circuit / fault on a DC branch DCA1, DCA2, DCA3, or the DC bus DCB. Alternatively, a connection for an external power supply for the control device and, if applicable, other devices of the DC switch Sx can be provided.

[0071] The control device is designed in such a way that (in particular periodically) the direction of the current and the current level are determined, if a first threshold value of the current level is exceeded, i.e. a current threshold value, the current flow through the switching device SCH is interrupted, if the current is in the reverse direction shortly before or during the exceeding of the first threshold value of the current level: after the interruption of the current flow, the DC bus side voltage level is compared with the DC branch side voltage level (periodically) and if the voltage difference is smaller than a voltage difference value, the switching device is switched to conductive. For example, this voltage difference value can be 10 volts. IeIf the voltage difference between the DC bus DCB and the DC branch DCA is less than 10 volts, the switching device SCH is switched on again by the control device SE; the switching device SCH becomes conductive for the flow of electrical current.

[0072] In addition, the control device SE can be further configured such that, if the current rises above a current rise threshold, the current flow through the switching device is interrupted. The invention can also be used after such an interruption, and if the current flows in the reverse direction, a comparison of the voltages and, if necessary, switching on can take place at the time of triggering / current interruption.

[0073] In the positive or negative conductor, before or after the switching device SCH, isolating contacts or a isolating switch can be provided in one or both conductors (positive or negative conductor) to galvanically isolate the DC device or DC switch. The isolating contacts can be switched by the control device SE.

[0074] Figure 3 shows an example of a switching device SCH with semiconductor switching elements. The switching device SCH has a series connection of a first and a second semiconductor switching element Q1, Q2. The first controllable semiconductor switching element Q1 is conductive, for example, for a first current direction, and the second controllable semiconductor switching element Q2 is conductive for the opposite current direction.

[0075] A first diode D1, which is conductive in the opposite current direction as the first semiconductor switching element Q1, is connected in parallel to the first semiconductor switching element Q1, and a second diode D2, which is conductive in the first current direction of the first semiconductor switching element Q1, is connected in parallel to the second semiconductor switching element Q2.

[0076] The switching device SCH is designed with two-pole connections (for positive and negative conductors); in the example, the first and second semiconductor switching elements Q1, Q2 are located in one conductor, in the example in the positive conductor; the negative conductor is fed through and has no semiconductor switching elements.

[0077] Alternatively, the semiconductor switching elements can also be arranged in the negative conductor or both conductors can have semiconductor switching elements.

[0078] The series connection of the two semiconductor switching elements Q1, Q2 is followed by an isolating contact on the device side or DC voltage branch side, whereby a first isolating contact TK1 is provided for the positive conductor and a second isolating contact TK2 is provided for the negative conductor, generally referred to as isolating contact or isolating contacts, for the galvanic isolation of the DC voltage branch or device.

[0079] The switching device SCH can also be constructed differently, for example, by connecting two semiconductor switching elements Q1, Q2 in parallel. A diode can be connected in series with each of the semiconductor switching elements Q1, Q2, and this series connection can be connected in parallel. Other variants are also conceivable.

[0080] In the following, the invention will be presented again in other words.

[0081] In DC networks with distributed capacitances in the load feeders and feeder paths, the protection of machines and equipment can now be achieved using power electronic DC switches (switches). The switches are expected to be selective. Selectivity now requires that only the faulty feeder is (permanently) switched off, while all other branches remain active.

[0082] Semiconductor-based switching elements such as Si IGBTs or SiC MOS-FETs reach a saturation current that is smaller than the short-circuit currents in the event of a fault. Since these switching elements can only withstand such a saturation current for a short time (in the ps range), these switches must switch off for self-protection, regardless of the required selectivity in such a DC voltage system.

[0083] According to the invention, two possibilities for realizing selectivity are proposed: A: Evaluation of the short-circuit current and voltage before the switch is switched off, B: Evaluation of the voltage after the switch is switched off and, if necessary, rapid re-switching on.

[0084] In DC voltage networks with distributed capacitances and driving line inductances, bidirectional, power electronic switches should be used to protect the outgoing circuits. These switches can be equipped with current and voltage measurement (not only emitter-collector, but also between the poles) for self-protection. This allows the current, current rise rate, voltage, and voltage rate of change to be measured.

[0085] From the current direction, a DC voltage switch, such as an outgoing circuit breaker S1, can determine whether the fault has occurred in its DC voltage branch or outgoing circuit and whether it must switch off permanently, provided that the fault is reliably recognized as such from the current increase and voltage dip.

[0086] If a switch needs to shut down for its own protection, the above-mentioned variables can be recorded and buffered in the switch for evaluation by a microcontroller or controller. A control voltage is advantageously still available for the switch's operation.

[0087] For the second and third DC switches S2, S3 in adjacent feeders or outgoing circuits of comparable or lower power, the current flow in the event of a fault is in the reverse direction, towards the DC bus or the DC distribution, and it can additionally determine a time for reconnection, which will be in the millisecond / ms range, based on the depth of the voltage dip (discharge of the capacitance in the outgoing circuit) and a recurring voltage through further feeding paths and line inductances. U s t = R L ⋅ I s t + L L d I s t / dt − ∫ I s t / C dt + UFehler

[0088] If switches need to be opened for self-protection, S2 and S3 can decide to reconnect as quickly as possible based on the current direction. In this case, S4 would detect a return of the DC voltage and, if it had already been opened, could also reconnect very quickly. S1 would remain open due to the short circuit and thus the lack of voltage at the outgoing circuit.

[0089] The invention has the advantage that, compared to protection with electromechanical circuit breakers, selectivity can be ensured in capacity-buffered DC voltage networks if power electronic switches do not switch off or switch on again according to system-adapted criteria and can continue to supply the non-faulty outgoing circuits before the capacities are completely discharged and the system has to be restarted.

[0090] In particular, the criteria take into account not only the flowing current, but also the voltage and its respective changes. Rapid switching on and off using power electronic switches is crucial for this.

[0091] The advantage is improved selectivity without oversizing the semiconductor switches.

Claims

1. DC voltage switch (Sx) for coupling a DC voltage branch (DCA, DCA1, DCA2, DCA3) to a DC voltage bus (DCB), said DC voltage branch being provided for at least one DC voltage device (Gx) and having a positive conductor and a negative conductor; wherein in a forward direction the electric current flows from the positive conductor (DCP) of the DC voltage bus (DCB) via the DC voltage switch (Sx) to the positive conductor of the DC voltage branch (DCA) and from the negative conductor of the DC voltage branch (DCA) via the DC voltage switch (Sx) to the negative conductor (DCN) of the DC voltage bus (DCB), and in a reverse direction the electric current flows from the positive conductor of the DC voltage branch (DCA) via the DC voltage switch (Sx) to the positive conductor (DCP) of the DC voltage bus (DCB) and from the negative conductor (DCN) of the DC voltage bus (DCB) via the DC voltage switch (Sx) to the negative conductor of the DC voltage branch (DCA); the DC voltage switch (Sx) comprising: a semiconductor-based, electronically controllable switching device (SCH), a DC voltage bus-side voltage sensor (U1) provided upstream of the switching device (SCH), for determining the DC voltage bus-side (DCB) voltage level, a DC voltage branch-side voltage sensor (U2) provided downstream of the switching device (SCH), for determining the DC voltage branch-side (DCA) voltage level, a current sensor (I) for determining the current level and the current direction, characterized by a control device (SE) connected to the switching device (SCH), the voltage sensors (U1, U2) and the current sensor (I), wherein the control device (SE) is configured in such a way that - the direction of the current and the current level are determined, - in the event of a first threshold value of the current level being exceeded, the current flow through the switching device (SCH) is interrupted, - if, in the event of the first threshold value of the current level being exceeded, the determined direction of the current is the reverse direction: - after interruption of the current flow, the DC voltage bus-side (DCB) voltage level is compared with the DC voltage branch-side (DCA) voltage level and the switching device (SCH) is switched into the on state in the event of a voltage difference that is less than a voltage difference value.

2. DC voltage switch (Sx) according to Patent Claim 1, characterized in that the switching device (SCH) comprises at least one, in particular two, semiconductor switching element(s) (Q1, Q2) carrying the positive or negative conductor-side current flow.

3. DC voltage switch (Sx) according to Patent Claim 2, characterized in that the semiconductor switching element (Q1, Q2) is an insulated gate bipolar transistor, a metal oxide semiconductor field effect transistor or a gallium nitride transistor.

4. DC voltage switch (Sx) according to Patent Claim 2 or 3, characterized in that a diode (D1, D2) is connected in parallel with the semiconductor switching element (Q1, Q2), in particular with the forward direction of said diode counter to the forward direction of the semiconductor switching element (Q1, Q2).

5. DC voltage switch (Sx) according to Patent Claim 4, characterized in that the diode (D1, D2) has a low forward voltage, and in particular is a network diode or a Schottky diode.

6. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the DC voltage switch (Sx) is arranged in a housing (GEH) .

7. DC voltage switch (Sx) according to Patent Claim 6, characterized in that the housing (GEH) comprises a positive conductor input connection (PE), a negative conductor input connection (ME), a positive conductor output connection (PA) and a negative conductor output connection (MA); in that the input connections (PE, ME) are connectable to the DC voltage bus (DCB); in that the output connections (PA, MA) are connectable to the DC voltage branch (DCA); in that either the switching device (SCH) is provided between positive conductor input connection (PE) and positive conductor output connection (PA) or / and the switching device (SCH) is provided between negative conductor input connection (ME) and negative conductor output connection (MA).

8. DC voltage switch (Sx) according to Patent Claim 7, characterized in that the connections not comprising the switching device (SCH) are connected to one another by an electrical line.

9. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the voltage difference value is 10 volts.

10. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the DC voltage switch (Sx) is an outgoer switch or is used in an outgoer switch.

11. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the current sensor (I) is a Hall effect-based sensor.

12. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the control device (SE) is furthermore configured in such a way that in the event of a rise in the current that exceeds a current rise threshold value, the current flow through the switching device is interrupted.

13. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that a power supply unit, an energy store or a connection for an external energy supply is provided for supplying energy to the control device (SE).

14. DC voltage switch (Sx) according to any of the preceding patent claims, characterized in that the control device (SE) comprises a microprocessor.

15. DC voltage branch (DCA) comprising a DC voltage switch (Sx) according to any of Patent Claims 1 to 14, wherein the DC voltage switch (Sx) is connected to a DC voltage bus (DCB) at one end and to devices (Gx), in particular loads, at the other end.

16. Method for coupling a DC voltage branch (DCA, DCA1, DCA2, DCA3) to a DC voltage bus (DCB), said DC voltage branch being provided for at least one DC voltage device (Gx) and having a positive conductor and a negative conductor; wherein in a forward direction the electric current flows from the positive conductor (DCP) of the DC voltage bus (DCB) via a DC voltage switch (Sx), with a switching device (SCH), to the positive conductor of the DC voltage branch (DCA) and from the negative conductor of the DC voltage branch (DCA) via the DC voltage switch (Sx) to the negative conductor (DCN) of the DC voltage bus (DCB), and in a reverse direction the electric current flows from the positive conductor of the DC voltage branch (DCA) via the DC voltage switch (Sx) to the positive conductor (DCP) of the DC voltage bus (DCB) and from the negative conductor (DCN) of the DC voltage bus (DCB) via the DC voltage switch (Sx) to the negative conductor of the DC voltage branch (DCA); wherein the DC voltage bus-side (DCB) voltage level is determined, wherein the DC voltage branch-side (DCA) voltage level is determined, characterized in that the current level and the current direction are determined, in the event of a first threshold value of the current level being exceeded, the current flow through the switching device is interrupted, if, in the event of the first threshold value of the current level being exceeded, the determined direction of the current is the reverse direction: after interruption of the current flow, the DC voltage bus-side (DCB) voltage level is compared with the DC voltage branch-side (DCA) voltage level and the switching device (SCH) is switched into the on state in the event of a voltage difference that is less than a voltage difference value.